The present invention relates to a voltage-controlled variable oscillator, in particular for phase-lock loops.
As is known, a voltage-controlled oscillator (VCO) is an oscillator the frequency whereof varies according to the variation of an externally applied voltage. Said variable oscillators are extensively used in phase-lock loops (PLL), for example for conversion to video medium frequency. In particular, integratable variable oscillators, with a central frequency of 38.9 MHz and a variation interval of .+-.500 KHz, are currently required for this application. The oscillator is furthermore required to provide a signal which must be as stable as possible in terms of frequency and peak-to-peak value as the temperature and the process parameters vary.
In order to obtain the variability of the operating frequency, all current circuits use a capacitor in resonance with a fixed inductance. A known solution (shown in FIG. 1) has voltage-controlled varactors connected in parallel to the resonating circuit. These varactors can be internal or external to the integrated circuit; however, the integration of these varactors has problems in high-speed processes since generally they cannot be integrated conveniently. On the other hand, the use of external components necessarily entails a more complicated application which in some cases is therefore undesirable.
Another known solution provides the use of current-controlled saturation capacitors obtained within an integrated circuit (see FIG. 2). According to this solution, by varying the current I, it is possible to directly bias the collector junctions of the transistors, thus modulating the equivalent capacity of the transistors themselves. However, this solution, despite being easily integratable, has numerous disadvantages related to the fact that the peak-to-peak output value depends on the operating frequency, the output frequency depends on the process parameters (e.g. on the inverse transit time) and the output characteristics depend on the temperature (through the saturation characteristics of the transistors).